Photochromic glasses, based on silver halide crystals, can be rendered polarizing, by thermally softening and stretching the softened glass. This elongates the silver halide crystals and generates a form birefringence in the glass. A permanent polarizing effect can be obtained by heat treating the stretched glass in a reducing atmosphere. The elongated silver halide crystals are at least partially chemically reduced to elongated particles of metallic silver.
Glass polarizers, produced from silver halide-containing glasses as just described, have enjoyed commercial success in applications employing radiation in the infra-red portion of the spectrum. However, the parallel and perpendicular absorption curves reverse in magnitude and cross over in the visible wavelength region at about 480 nm. This has prevented the silver halide glasses from being considered the use as a visible polarizer, that is, for applications that require polarization in the same direction across the entire visible wavelength region of the spectrum. As used in this application, the term "visible polarizer" means a polarizer that provides effective polarization of radiation across the entire visible wavelength region of the spectrum. This is essentially the wavelength range of 400-700 nm.
U.S. Pat. No. 3,325,299 (Araujo) discloses phototropic (now known as photochromic) glasses in which copper and/or cadmium halide crystals are precipitated to impart photochromic behavior. The disclosed glasses have R.sub.2 O-Al.sub.2 O.sub.3 -B.sub.2 O.sub.3 -SiO base compositions. These oxides total at least 85% of the glass composition. Copper and/or cadmium oxides, together with sufficient chlorine, bromine and/or iodine to react stoichiometrically with at least 0.3% copper and/or cadmium, are added to the base glass composition.
The family of glasses containing a cuprous and/or cadmium halide crystal phase are strongly absorbing in the ultraviolet region of the radiation spectrum. This is due to a sharp spectral cut-off of radiation at wavelengths shorter than about 400 nm. This is also in contrast to the photochromic glasses containing silver halide crystals as their active element. Such glasses do not have a sharp cutoff.
Cadmium is not required to secure absorption of ultraviolet radiation, and imparts no exceptional benefit to the glass. The halide crystals are effective to absorb ultraviolet radiation, but cadmium is known to be extremely toxic. Accordingly, its presence is severely restricted, and it is preferably completely avoided. Therefore, while cadmium halide glasses are within the scope of the present invention, reference throughout will be to copper halide glasses.
Copending application Ser. No. 08/270,052, now U.S. Pat. No. 5,517,356, and is also a continuation-in-part of Ser. No. 08/166,833. That Patent is directed to a glass polarizer that provides effective polarization of radiation across the entire visible wavelength region the spectrum. The glass is an R.sub.2 O-Al.sub.2 O.sub.3 -B.sub.2 O.sub.3 -SiO.sub.2 glass containing copper halide crystals. To produce the desired polarizing effect, copper halide crystals are precipitated in the glass, after which the glass is thermally softened and stretched. This also elongates and orients the crystals. Subsequently, the glass is fired in hydrogen to at least partially reduce the copper halide to metallic copper.
Heretofore, polarizers useful across the entire visible wavelength region of the spectrum, that is, 400-700 nm, have been produced from plastic materials. The disadvantages of such materials are well known. They cannot withstand elevated temperatures, scratch easily, bleach under intense light, and may undergo distortion under pressure.
It would be desirable to embody these visible polarizing and UV-absorbing characteristics in an ophthalmic lens. The present invention meets that need. It makes available an ophthalmic lens of photochromic glass that provides effective polarization across the entire visible wavelength portion of the spectrum. The glass lens has a sharp cutoff in transmission at about 400 nm, which removes harmful ultra-violet rays. This is of particular value in sunglasses to protect the eyes. The invention further provides a method of producing such a lens from glasses containing copper halide crystals.